The interplay of vibronic and spin-orbit coupling in the fluorescence quenching in trans-dithionated PDI

Organic chromophores such as the thionated derivatives of perylene diimides (PDIs) show prolonged triplet-excited state lifetimes in contrast to their pristine parent PDI molecule, which shows near unity fluorescence quantum yield. The excited state dynamics in the trans-dithionated PDI (S2-PDI) are studied here. Unlike PDI, the photo absorbing pp* state of S2-PDI is in close proximity to quasi-degenerate np* states. The latter exhibits an interesting vibronic problem leading to the breaking of orbital symmetry mediated through non-totally symmetric vibrations. The time-dependent quantum dynamics are studied with a diabatic model Hamiltonian involving three singlet and three triplet states coupled via 22 vibrational modes. A combined effect of multiple internal-conversion and inter-system crossing (ISC) pathways leads to population transfer from the (1)pp* state to the (3)pp* state via the np* states, with an overall ISC rate of 0.70 ps that compares well with the experimental value. The calculated absorption spectra for PDI and S2-PDI reproduce the essential vibronic features in the observed experimental spectra. The dominant vibronic progressions are found to have significant contributions from the vinyl stretching modes of the PDI core.

Automated summary

This paper investigates the excited state dynamics of the thionated derivative of perylene diimides (PDIs), S2-PDI. The study reveals the population transfer from the (1)pp* state to the (3)pp* state through multiple pathways, including internal conversion and intersystem crossing. The calculated results agree well with experimental observations.